when you migrate to another ISP, do you have to keep the same speed? I am guessing that BT would need to reconnect your line at the exchange, thus another fee. would I be correct in saying this?
yes, the speed remain the same when you migrate to other isp but sometimes BT do test your line again to ensure u get the speed quality on your line like noise margin can be changed. No, BT not need to go to exchange as it can be done at their switchboard network at BT.